How to allow negative NET price on Sales Order/Deliveries/Invoices?

Hello, we have the requirement to allow negative NET pricing on certain lines on Sales Orders, Deliveries and Invoices without using the discount fields. I've been able to enter a negative amount on the GROSS price for a line item, but then Sage makes that line item's NET price zero instead of allowing the negative. Is there an option to turn this off somewhere, and allow the negative to carry through to the NET? The overall net of the document will always still be positive, just the single line item needs to be negative.